The present invention is related to electro-thermal ice protection systems for aero structures, in general, and more particularly, to a system comprising a metal foil heater, including an integral parting strip, configurable to cover at least a portion of a leading edge of an airfoil; and a controller for controlling heating energy to the metal foil of the heater in accordance with a pulse duty-cycle and for controlling heating energy to the parting strip to maintain an air-stagnation zone of the leading edge virtually free of ice formation.
Currently proposed electro-thermal de-icing or anti-icing products for leading edge ice protection of aero surfaces generally utilize heating elements or electrodes disposed at a leading edge surface of the aero structure in the form of a serpentine or interdigitated finger area grid to deliver heating power to ice which may be formed thereon. One such proposed electro-thermal ice protection product operates in an electro-chemical mode in which energy is delivered through capacitive coupling to the accreted ice by high frequency (HF) audio electro-chemical heating. Through an electrolysis reaction, gases are generated at the ice-surface interface which tend to affect the ice adhesive bond and separate the ice from the surface. When disposed on an airfoil under flight conditions in a wind tunnel, it was found that the electrolysis reaction was highly dependent on temperature, i.e. much more active as the temperature rose. Accordingly, heat energy tended to concentrate at a spot or spots in the heater grid where the temperature rose and the warmer the spot the more energy was drawn to it creating an avalanche effect. So, a spot on the heater grid where the electro-chemical reaction first started ended up drawing heat energy away from the rest of the grid to the point where ice could no longer be removed from the colder surfaces.
In addition, it was found that the electro-chemical heating method fails to perform at low temperatures, like below −20° C., for example, because the ice which is a protonic semiconductor is not conductive enough at these temperatures to allow significant power to be imparted at voltage levels that are consistent with safe aero design practice as dictated by the U.S. FAA regulations and FAA advisory circular bulletins such as AC 25.981, for example.
Further, conductive heater electrodes have been proposed to be disposed on, but insulated from, the aero surfaces to be protected from ice in parallel finger patterns which ran the length of the heater. In one serpentine pattern design, each finger had a width dimension of 0.020 inches or 20 mils with a separation distance from adjacent fingers of 20 mils, and a thickness dimension of 0.004 inches or 4 mils. However, this bare metal surface heater arrangement was found not to be structurally sound when tested under airfoil flight conditions, especially under rain erosion and particulate abrasion conditions. Any layer of material disposed over the heater grid for protection against rain erosion and particulate abrasion conditions will cause heat losses which will impose additional heating power requirements for ice protection. Generally, power supply systems on-board the aero structure can not afford to provide such ice protection power without affecting the power requirements to other systems.
A similar ice protection system is proposed in the U.S. Pat. No. 6,027,075, entitled “Systems and Methods For Modifying Ice Adhesion Strength”, and issued to Victor Petrenko on Feb. 22, 2000. Petrenko recognized that there is a liquid like layer (LLL) at the ice interface between the ice layer and aero structure which is a major factor in the ice adhesion strength. Petrenko realized that the LLL functions as a wetting surface which substantially increases the effective contact area between surfaces. The Petrenko system comprises a first electrode connected to the aero surface, preferably in the form of a grid, an electrically insulating layer disposed over the first electrode, and a second electrode formed over the first electrode in a similar grid as the first electrode. In operation, when ice is detected on the surface, a DC voltage is applied across the first and second electrode grids of sufficient magnitude to decrease the ice adhesion strength of the LLL and facilitate ice removal by the wind slipstream across the aero surface.
Since the Petrenko system proposes a similar grid of electrodes either in a serpentine or interdigitated design which couple energy into the ice layer accreted on an aero structure to loosen the bond between the ice and aero surface, it will suffer similar structural and power requirement drawbacks in an aero structure flight environment as described for the AC electro-chemical heating system above.
The present invention overcomes the aforementioned drawbacks of the present electro-thermal ice protection systems and offers an electro-thermal ice protection system rugged enough to withstand exposure to an aero structure operational environment, and capable of shedding ice from an aero surface at safe voltages and substantially reduced power levels, and before the ice accretes to a dangerous thickness level.